EP0012809B1 - Process for the preparation of powdery coating compositions for coating metallic objects according to the melt-coating process on the basis of polylaurolactam powder, and their use in producing coloured powders according to the dry-blend process - Google Patents

Description

The invention relates to an improved process for the preparation of coating compositions for coating metallic objects by the hot-melt coating process based on polylaurine lactam powder, which is obtained from polylaurine lactam granules, which is obtained by polymerizing laurolactam in the presence of 2 to 10 percent by weight of water, 0.3 to 0 , 7 percent by weight phosphoric acid at temperatures between 265 and 300 ° C in the absence of any chain stabilizers have been obtained under autogenous pressure.

It is known to use powdered coating compositions based on polyamides for the production of lacquer-like coatings on metals. The coating takes place according to the melt coating process. This includes the vortex sintering process, the flame spraying process or the electrostatic coating process.

A particularly advantageous method is described in DE-AS 2 545 267. In this respect, however, this way of working is still in need of improvement. According to the known method of working, it is necessary to adhere to two different work sequences for non-pigmented and pigmented coating powders. For example, the unpigmented so-called natural powders obtained by grinding must be re-condensed in a solid state, which reduces the space-time yield and causes higher energy costs.

The pigmented powders, on the other hand, require a completely different production method, namely production by reprecipitation from solvents. Consequently, two different production lines have to be set up for the production of the different powders, which requires a considerably higher outlay and has the disadvantage of handling solvents.

Finally, the powders obtained according to the prior art described are not yet satisfactory in their quality. Both the so-called natural powders and the pigmented powders show a strong drop in elasticity, especially after the so-called "dry-blend coloring" and when coating thicker metal parts. In addition, the powders have a low resistance to yellowing (overburning stability). This disadvantage manifests itself in that color differences occur when coating metal parts of different thicknesses. Both the natural powders and the color powders obtained from these powders by "dry-blend coloring" show this yellowing to different degrees.

The task was to find a simplified manufacturing process that is suitable for the production of both pigmented and pigment-free powders. Furthermore, the powders produced in this way should not have the disadvantages of reduced elasticity and yellowing described and, finally, the already existing good properties should not be lost.

This object is achieved if granules with a viscosity η _rel of 1.50 to 1.80 (measured in 0.5% m-cresol solution at 25 ° C.) are ground after precooling in liquid nitrogen at -100 to 0 ° C. , the powders classified by sieving and classifying to a particle size distribution of at least 99%> 40 microns and at least 99% <250 microns, being extracted before or after or after classification with methanol or ethanol at 25 to 100 ° C and at Manufacture pigmented powders, the pigments are added before or during the polymerisation: to the monomeric laurolactam or after the polymerisation before granulation to the polylaurine lactam.

The polylaurine lactam granules used are obtained by polymerizing laurolactam in the presence of 2 to 10 percent by weight of water and 0.3 to 0.7 percent by weight of phosphoric acid in the absence of any chain stabilizers at temperatures between 265 and 300 ° C. under autogenous pressure.

The granules used can be used in a wider viscosity range, namely η _rel from 1.50 to 1.80.

Compared to process A of DE-AS 2545267 there is also the advantage that the post-condensation stage can be omitted. Compared to process B of DE-AS, there is the advantage that the precipitation method can be omitted and that the pigments do not have to be added in a separate special process step.

Also with regard to the intermediate extraction stage, you are not tied to a specific sequence. Suitable pigments are e.g. Titanium dioxide, barium sulfate, iron oxide, cadmium pigments or carbon black which are added to the polyamide before granulation in conventional amounts of 1 to 20, preferably 2 to 10 percent by weight, before or during the polymerization or after the polymerization.

The optionally pigmented granules are milled in liquid nitrogen at temperatures of -100 to 0 ° C., preferably -70 to -20 ° C., preferably by impact crushing. After grinding, the coarse particles> 250 µm are screened off, for example, under an air jet sieve. The disruptive fine fraction <40 µm is removed using gravity classifiers, for example. The grain size distribution is therefore at least 99%> 40 µm and at least 99% <250 µm, in other words the coarse and fine fraction should be less than 1 ° / o.

Vortex sinter powders generally have a grain size distribution within this range of 30 to 300 μm, in particular 60 to 250 μm, electrostatic or flame spray powder such a, from 0 to 100, in particular from 0 to 80 microns. The smoke during the coating process is prevented by extraction with methanol or ethanol at 25 to 100 ° C, especially at the reflux temperature of the alcohol. At temperatures above the boiling point of the alcohol one works accordingly under pressure. The extraction can take place both in the granulate stage or by extracting the powders, which can be extracted both before and after the classification. After extraction, the granules or the powders are dried in the customary manner, for example by treatment in a paddle or tumble dryer at temperatures between 40 and 100 ° C. in an inert gas atmosphere or, if appropriate, by applying a vacuum. Aging, heat or light stabilizers, which are usually present in amounts of 0.1 to 2 percent by weight, such as sterically hindered phenols, triazines, phosphites, hypophosphorous acid, hypophosphites can be added during or after the polymerization or, for example, during the extraction or grinding process will.

The simplified method of working allows coatings, in particular by the fluidized bed process, to be produced without annoying smoke, which have a very good flow, a good edge coating, adhesion and excellent resistance to aqueous alkaline solutions. In addition, the coatings remain elastic even when coating thick metal parts and are very resistant to yellowing. Powders colored using the “dry blend process” have a much better mechanical property profile.

example 1

In an autoclave, 10 kg of laurolactam are heated together with 800 g of water and 60 g of 85% phosphoric acid at 280 ° C for 7 hours, a pressure of 19 bar being established. The pressure is then released and, when a viscosity of η _rel = 1.60 is reached, the product is expanded and granulated. The polymer particles are refluxed for 5 hours with twice the amount by weight of ethanol. Then 100 g of 4-hydroxy-3,5-di-tert-butyl-phenylpropionic acid hexamethylene bisamide dissolved in 1 gelöst ethanol are stirred in. After distilling off the solvent u. Drying, the granules are pre-cooled with liquid nitrogen (-190 ° C) and ground at -40 ° C. The powder portion> 250 µm is sieved with an air jet sieve, the portion <40 µm is separated with an air classifier.

The coating according to the vortex sintering method gives the following evaluation: no smoking when coating, smooth surface, good flow in cross-lattice points, very good coating of sharp edges, Erichsen cupping> 10 mm, impact depth> 1800 mm / 7.6 kg, resistance to 1 % boiling lye of a heavy duty detergent> 2000 hours, yellowing when coating a metal part at 260 ° C with the dimensions 14 x 25 X 80 mm: not detectable.

By adding 2 ° / o titanium dioxide pigment and 0.1% carbon black in the “dry blend process”, a gray powder is produced from the natural powder. When coating a metal plate with a thickness of 1 mm, a layer is obtained that does not tear when suddenly bent by 180 ° C.

Example 2

The procedure is as in Example 1, with the difference that 600 g of titanium dioxide are added before the polycondensation.

The coating with the white powder by the vortex sintering method gives the same evaluation as with the natural powder from Example 1. Here, too, no yellowing is found when coating a metal part at 260 ° C. with the dimensions 14 × 25 × 80 mm.

A yellow powder is produced by adding 2 ° / o cadmium yellow pigment using the «dry blend» method. When coating a metal plate with a thickness of 1 mm, a layer is obtained which does not tear if it suddenly kinks by 180 ° C.

Comparative Example 1: (DE-AS 2 545 267 method A)

In an autoclave, 10 g of laurolactam together with 800 g of water and 60 g of 85% phosphoric acid are heated to 280 ° C. for 7 hours, a pressure of about 19 bar being established. Then it is cooled to 240 to 230 ° C (pressure about 13 to 14 bar) and the bottom valve is opened. The polymer is passed into a water bath by a water jet that strikes the polymer strand at an angle of approximately 90 ° and, after cooling, is comminuted or crushed in a cutting mill. The polylaurin lactam is dried at a pressure of about 30 mbar and a temperature of 60 to 80 ° C. The solution viscosity is η _rel = 1.39.

The polymer particles are refluxed with twice the amount by weight of methanol for 2 hours. The methanol is drained and replaced with the same amount of fresh methanol. It is again boiled under reflux for 2 hours, the methanol is drained and dried at 30 mbar and 80 ° C. The polylaurin lactam is ground in a baffle plate mill at a circulating gas temperature of 20 ° C. The mill is kept at a temperature of 20 ° C. by the partial addition of liquid nitrogen. The powder is then post-condensed in a drying cabinet in a thin layer by passing nitrogen over it at a temperature of 145 ^° C. for 5 hours. The powder then has a solution viscosity of η _rel = 1.59.

The coating according to the vortex sintering method gives the following evaluation: no smoking when coating, smooth surface of the layer, good to very good coating of sharp edges, Erichsen cupping> 10 mm, Impact of> 1800 mm / 7, '6 kg, resistance to 1% boiling lye of a detergent> 2000 hours, yellowing when coating a metal part at 260 ° C with the dimensions 14 x 25 X 80 mm: very strong. A gray powder is made from the natural powder by adding 2% titanium dioxide pigment and 0.1% carbon black in the dry blend process. When a metal plate with a thickness of 1 mm is coated, a layer is obtained which breaks open when it suddenly kinks by 180 ° C. When coating a metal part at 260 ° C with the dimensions 14 X 25 X 80 mm, a strong yellowed coating is also obtained.

Comparative Example 2: (DE-AS 2 545 267 method B)

A polylaurine lactam prepared according to Comparative Example 1 with a solution viscosity of η _rel = 1.60, which had been crushed and extracted, was used.

100 g of this polylaurine lactam, 200 ml of ethanol and 7 g of titanium dioxide pigment are placed in a glass autoclave. The mixture is heated to 140 ° C. for 2 hours, a pressure of about 7 bar being established. The mixture is then cooled with a stirring speed of about 5 rpm at 1 ° C./min. The powder is dried in a drying cabinet at 0.7 mbar and 80 ° C. When coating metal parts, the white colored powders have the same evaluation in all points as for the natural powders of Comparative Example 1. A strong yellowing is also found here when coating a metal part at 260 ° C with the dimensions 14 X 25 x 80 mm.

A yellow powder is produced by adding 2% cadmium yellow pigment using the dry blend method. When coating on a metal plate with a thickness of 1 mm, a layer is obtained which breaks by 180 ° C when suddenly bent.

The results from the examples and comparative examples are summarized in the following table.

Claims (2)

1. A process for the preparation of a powdery coating composition for coating metallic objects by the melt-coating process, the composition being based on polylaurolactam powder which has been obtained from polylaurolactam granules which in turn have been obtained by polymerisation of lauric acid lactam in the presence of 2 to 10 weight percent of water and 0.3 to 0.7 weight percent of phosphoric acid at a temperature of from 265 to 300°C in the absence of any chain stabilisers at autogenous pressure, characterised in that for the preparation of the composition not only pigmentfree polylaurolactam granules but also pigmented polylaurolactam granules are used ta a viscosity η_rel of 1.50 to 1.80 (measured in 0.5% m-cresol solution at 25°C) and are ground after cooling in liquid nitrogen at a temperature from -100 to 0°C, and the powder is classified by seperating coarse and fine particles to provide a particle size distribution of at least 99% > 40 µm, and at least 99% < 250 µm, the powder being extracted with methanol or ethanol at 25 to 100°C before the grinding or before or after the classifying and in the case of the preparation of pigmented powder the pigment being incorporated into the polylaurolactam before or during the polymerisation of the monomeric lauric acid lactam or after the polymerisation but before the granulation.

2. The use of the coating powder obtained by the process according to claim 1 for the preparation of pigmented powder by the «dry-blend process».